A conventional object detecting apparatus mounted on a vehicle uses a laser light, for instance, to detect a distance to an object such as a preceding vehicle. This detecting apparatus periodically drives a laser diode to radiate the laser light towards the forward area of the vehicle, and detects the laser light reflected from the forward object by a photo sensor. The detecting apparatus measures the distance to the forward object based on a time difference between a light radiation time and a light reception time.
Specifically, as disclosed in JP 2002-031685A, the detecting apparatus comprises a light radiation unit for radiating a laser light, a polygon mirror and a light receiver unit for receiving a reflected laser light. The polygon mirror is shaped in a frustum of a hexagonal pyramid and rotatable as a scanning mirror.
This detecting apparatus is used in a harsh environment of a vehicle. Therefore, the above scanning parts, optical components, electronic circuits and the like are accommodated in a case to be protected from frosting of water or foreign material such as dust. The case has a light radiating window and a light receiving window, and those windows are air or fluid-tightly covered with transparent plates, respectively. Those transparent plates, however, may be subjected to flying stones, etc, which will break the transparent plates.
If the detecting apparatus is continuously used to detect the object or distance with the broken transparent plates, various foreign matters enter the case and various components inside the case will rust due to frosting. As a result, accurate object detection will be disabled.